The most important suggestion is to focus a few islands worth of work on varying and increasing content near London, so that the player nearly always has new prose to read and new world-building to become a part of whenever he returns to an early-game port.
ANOTHER SUGGESTION
Create more missions that do not involve returning to London.
Some of the admiralty’s contacts may, for example, get the player involved in conflicts between spy networks. They may start sending him on missions independently, they may introduce him to more other folks with their own interests, etc.